The invention relates to digital loop transmission systems employing optical fibers, and in particular to a means for testing wire pairs which extend beyond the last fiber link into subscriber premises.
Optical fiber has become the medium of choice for sending digital signals between the central office and remote terminals. Recently the telephone service providers have extended the use of fiber optics beyond the remote terminal by setting up distant terminals on or near the subscriber premises and connecting these distant terminals to the remote terminal by a fiber optic link. The distant terminal converts the digital signals from the fiber into normal analog signals. Beyond the distant terminals, wire pairs, or "drop" wires extend into the customer premises to provide service. While a fiber optic link between remote and distant terminals will greatly increase the information capacity to the subscriber, it also creates a problem in that it eliminates electrical access to the customer's wire pair for determination of fault conditions.
The present method for electrical testing of the subscriber's line utilizes a wire pair extending between the central office and remote terminal which can by-pass the digital link when testing is required (see, for example, U.S. Pat. No. 4,270,030 issued to S. J. Brolin, et al). This technique is acceptable where thousands of customers are being serviced, as is usually the case between the central office and remote terminals. However, it becomes prohibitively expensive to provide wire pairs along with each fiber out to the distant terminals.
It is, therefore, an object of the invention to provide a means for testing the wire pair to customer premises in systems employing optical fiber up to the distant terminal.